Certain types of dietary fiber may be protective against asthma. Experts note the incidence of asthma is increasing in less developed countries, as people there shift their eating habits away from high-fiber foods in favor of processed ones.

We get two types of dietary fiber from food--insoluble and soluble. Insoluble fiber, from foods like whole grains, cucumbers, and broccoli, includes so-called roughage, which helps cleanse the bowel.

A new study by researchers in Switzerland suggests that soluble fiber, which comes from fruits and vegetables, and is broken down by microbes in the intestines, may reduce inflammation in the lungs.

Benjamin Marsland, an immunologist at the University of Lausanne, says that until recently, asthma was virtually unknown in nations where people eat a lot of soluble fiber. "In some countries, Burkina Faso, for example, dietary fiber intake is very high and there is no development of allergies, whereas in westernized countries we have an intake of dietary fiber is low and the allergies have been increasing," Marsland said.

Gut bacteria break down soluble fiber, which includes pectin from apples, berries, and citrus, into short-chain fatty acids. Marsland says the fatty acids interact with spongy tissue inside the bones, where protective immune cells are produced, and help quiet immune system overactivity. An overzealous immune reaction can lead to inflammation.

To see whether dietary fiber could influence disease development outside the digestive tract, such as asthma, Marsland and colleagues studied two groups of rodents. One group of mice had been fed a diet high in soluble fiber for two weeks while the other group was fed a diet low in pectin. Both groups were then exposed to dust mites, a leading cause of asthma, a condition marked by lung inflammation, narrowing of the airways, and wheezing.

Marsland says the mice that ate less soluble fiber had strong allergic reactions to the dust, including the presence of inflammatory compounds in the lungs and constricted airways similar to what's seen in people. The mice that consumed food rich in pectin, according to Marsland, had lower levels of the immune cells that are usually elevated in allergic asthma.

"So the mechanism through which diet is helping the lung is the dietary fiber changes the bacteria in our intestinal tract, which changes the metabolites in our circulation. This is influencing how our immune cells develop," he said.